Stress in subterranean formations are usually determined in order to design formation fracturing operations, but typically these stresses are determined empirically by applying pressure to the formation from a wellbore until a fracture initiates. Typically, formation stresses will not be important variables in design of wellbore tubulars because the tubular strength is dictated by the necessity of the tubular to support a significant length of itself. This is not the case when the wellbore is to be used as a heat injection well in a thermal recovery project.
The casing will only have to support itself until it is cemented into place. This is done when the casing is relatively cool. When the heat injection well is placed in service, the casing will be heated to a temperature that is preferably between about 1400.degree. F. and 2000.degree. F. The thickness of the casing must be sufficiently thick so that, at these conditions, the casing will not buckle due to formation stress.
Even if the initial formation stress is determined prior to beginning heating operation of a heat injection well, the initial stress may not be indicative of the stress over the entire cycle of the heating operation. Measurements of stress in on certain rocks as cores of the rocks are heated up in a constrained volume show a large initial increase in compressive stress. A method to monitor such increase in stress is desirable in order to prevent collapse of a casing as heat is injected into the formation from a heater in the casing. For example, the operating temperature of the well may be limited initially if the formation stress increases initially, and then the operating temperatures might be increased later in the process if formation stresses decrease.
It is therefore an object of the present invention to provide a method to determine to operate heat injection wells wherein the stress within a formation during the operation of a wellbore is determined, and operating temperature limitations are adjusted according to measured formation stresses.